Generally, when making a finished photosensitive resin printing plate carrying images, a photosensitive resin printing plate stock cut into a required size is set on an image exposure table. Then, the uppermost cover film layer is removed, and an original drawing film, such as a negative film, is vacuum adhered to the photosensitive resin layer and exposed to an active light.
With the increasing demand for a rubber-like flexible photosensitive resin relief used in the business form printing field in recent years, the photosensitive resin printing plate stock tends to be made more flexible. In generality, a smaller hardness of the photosensitive resin printing plate stock results in greater tackiness of the surface of the photosensitive resin layer.
When the surface of the photosensitive resin layer has greater tackiness, air intrudes between the surface and the original drawing film when they are adhered to each other, so that a uniform contact between them is prevented and the image reproducibility is impaired.
In addition, when the original drawing film is removed from the photosensitive resin layer after exposure to light, the tackiness causes separation failure, thus damaging the both. A typical method to solve this problem involves formation of a thin layer (tackiness preventive layer) of a polymer that imparts less tackiness on the surface of the photosensitive resin layer. For example, Japanese Patent Unexamined Publication No. 49803/1976 (polyvinyl alcohol having a saponification degree of not less than 90%), Japanese Patent Unexamined Publication No. 110010/1977 (polyvinyl alcohol having a polymerization degree of not less than 800 and saponification degree of 70-90%), Japanese Patent Unexamined Publication No. 68224/1979 (water-soluble polymer), Japanese Patent Unexamined Publication No. 110941/1981 (soluble polyamide) are known.
When the photosensitive resin printing plate stock is used for preparing a finished flexographic printing plate, a tackiness-free coating layer (tackiness preventive layer) is highly required, since the photosensitive resin layer is more flexible and has higher tackiness. In particular, it is associated with a propensity toward occurrence of wrinkles on the tackiness preventive layer. Thus, Japanese Patent Unexamined Publication No. 208556/1982 discloses a technique wherein a releasing layer is formed on the tackiness preventive layer of a finished flexographic printing plate and US Patent No. 5077175 discloses a technique wherein a plasticizer is added to polyvinyl alcohol for the prevention of wrinkles when forming a tackiness preventive layer. The present inventors have filed a patent application drawn to a tackiness preventive layer composition for flexographic printing, which comprises polyvinyl alcohol and glycerin in a proportion of not less than 4 wt % to less than 8 wt % of the entire composition (Japanese Patent Unexamined Publication No. 130325/1992). Also, they have filed a patent application drawn to a tackiness preventive layer and a composition thereof for a photosensitive resin printing plate stock, which contains polyvinyl alcohol and a crystallizing agent soluble in water and having a boiling point of 120.degree. C.-240.degree. C. (Japanese Patent Unexamined Publication No. 324497/1994).
However, formation of a releasing layer as disclosed in Japanese Patent Unexamined Publication No. 208556/1982 not only increases production steps but also tends to produce coating defect due to the application of a tackiness-free coating layer on the releasing layer, thus requiring high coating technique. The polyvinyl alcohol containing a plasticizer disclosed in U.S. Pat. No. 5,077,175 contains a low molecular weight polyethylene glycol in a proportion of at least 8% of the entire weight of the composition. However, this mixture is susceptible to influence of external factors such as humidity, and the tackiness preventive layer comes to have tackiness under high humidity, which is due to moisture absorption, to impair the original object of non-tackiness.
Japanese Patent Unexamined Publication No. 130325/1992 is drawn to a composition having a certain glycerin content, and somewhat effective for preventing wrinkles of a tackiness preventive layer, but the effect thereof is degraded when the handling work environment changes. In addition, mist (dew condensation) of glycerin is developed during production, since glycerin has a low molecular weight, thus polluting production apparatuses. What is more, glycerin is said to be harmful to human. When coating temperature etc. change, moreover, a glycerin content also varies, which in turn results in difficulty in producing products having a constant quality, thus posing various problems in terms of production and quality.
On the other hand, Japanese Patent Unexamined Publication No. 324497/1994 has resolved the above-mentioned problems encountered during production, whereas could not entirely prevent wrinkles of the tackiness preventive layer. In other words, wrinkles were sometimes observed when, for example, a photosensitive resin printing plate stock was handled roughly, plate making environment changed or the photosensitive resin layer became more flexible.
As mentioned earlier, none of the prior art techniques is suitable for the tackiness preventive layer of flexible photosensitive resin printing plate stocks. In particular, the problem of wrinkles should be solved to meet the requirements of quality. For example, when separation of a cover film is stopped and then resumed, linear, stripe wrinkles are developed at the site where the separation was stopped, so that the cover film is generally required to be separated at once without interruption. Yet, it is extremely difficult to completely prevent stripe wrinkles, since the separation may be stopped due to an incorrect operation for separation and the like.
In other cases, greater sizes of the photosensitive resin printing plate stock could lead to rough handling of the stock due to increased difficulty in handling, in which case the photosensitive resin printing plate stock may be deformed. When the deformed photosensitive resin printing plate stock is set on a normal, flat image exposure table, and the cover film is peeled off, the tackiness preventive layer on the photosensitive resin layer could have developed wrinkles. Moreover, when the photosensitive resin printing plate stock is deformed into a shape having irregularities and then returned to the original plate shape, the tackiness preventive layer thereof should have produced many wrinkles which do not disappear by simply leaving the plate stock.
When a printing plate (relief) prepared using a photosensitive resin printing plate stock having wrinkles in the tackiness preventive layer is used for printing, the resulting printed matter shows defective parts due to the wrinkles, whereby the quality of the printed matter is degraded and the product value is lost. As such, the photosensitive resin printing plate stock requires sufficient caution in handling and transport thereof, though in fact the realization thereof is difficult in actual handling.
Moreover, a photosensitive resin printing plate stock cannot be stored or made into a printing plate under a certain constant environment throughout a year, and therefore, it is required to be able to retain certain quality even when the environment has changed. In the case of a tackiness preventive layer mainly composed of a water-soluble polymer, in particular, its water content tends to vary due to environmental differences, which is construed as producing wrinkles and degrading the quality thereof.
It is therefore an object of the present invention to solve the above-mentioned various defects of the conventional techniques, whereby to avoid occurrence of wrinkles in the tackiness preventive layer of the photosensitive resin printing plate stock during any handling or under any environment.